Hello there, coffee-bean!
Thanks for using Apple Support Communities.
I understand that when you connect to your car's Bluetooth, it automatically starts playing the last song you were listening to in your radio app, and it does this even though you have Force Quit the radio app. I listen to music on my iPhone all the time, so I can definitely partner with you and help find the best resolution.
The last song, or media, that you have played on your iPhone is always present in the "currently playing audio" section of Control Center. Even if you have quit the app, the media will still be available there for you to play. For more information on this, please see the Control Center section of the iPhone Help guide, specifically, this section:
Control Center gives you instant access to the camera, calculator, AirPlay, control and playback of currently playing audio, and other handy features. You can also adjust the brightness, lock the screen in portrait orientation, turn wireless services on or off, and turn on AirDrop. See AirDrop.
Open Control Center. Swipe up from the bottom edge of any screen (even the Lock screen).
Open the currently playing audio app. Tap the song title.
Close Control Center. Swipe down, tap the top of the screen, or press the Home button.
Turn off access to Control Center in apps or on the Lock screen. Go to Settings > Control Center.
At this time, there isn't an option to clear the currently playing audio from the Control Center. I would refer to your car/stereo owner's manual and see if there's an option to prevent automatically playing when synced with your iPhone.
Best Regards.
